Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot with force sufficient to create plastic deformation. Shot peening is often used as part of a machine process (e.g., metal forming) to improve fatigue life of a part. Shot peening can be useful in extending the fatigue life, stress corrosion resistance, and load carrying capacity of metal components. For example, fatigue failures may occur in areas of tension, thus by creating compressive residual stresses on the surface of the workpiece, the fatigue life can be enhanced.
Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Shot peening is often called for in the aerospace industry to relieve tensile stresses built up in the grinding process and replace those stresses with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, shot coverage; the shot peening process can significantly increase fatigue life of a component. Plastic deformation induces a residual compressive stress in a peened surface, along with tensile stress in the interior. Surface compressive stresses confer resistance to metal fatigue and to some forms of stress corrosion. The tensile stresses deep in the part are not as problematic as tensile stresses on the surface because cracks are less likely to start in the interior.
The beneficial effects of shot peening are mainly due to the residual stress field caused by the plastic deformation of the surface layer of material (e.g., a workpiece) resulting from the multiple impacts of the shot. Shot peening variables can affect performance of the final workpiece. Thus, it is important to know the values of the residual stresses in order to predict the mechanical strength of the peened parts and to know how these stresses vary by changing the shot peening parameters. By monitoring and controlling the parameters of the shot peening machine process, a consistent result can be obtained, while uncontrolled or uncalibrated shot peening can result in a widely scattered range of residual stresses, even cancelling out the benefits of shot peening or resulting in distortion of the workpiece.
Distortion is an undesirable affect of residual stress that may occur in a workpiece following a shot peening process. Distortion may be characterized as in-plane distortion or out-of-plane distortion. In-plane distortion includes expansion or contraction of the workpiece along a direction parallel to the plane of the workpiece surface. Out-of plane distortion includes displacement in the form of twisting and/or bending of the workpiece surface along a direction perpendicular to the surface.
Although the depth of shot peening-induced residual stress in a workpiece is typically shallow (e.g., 0.004 to 0.020 inch), out-of-plane distortion has a more noticeable effect on relatively thin metallic cross-sections that are less resistant to bending as compared to thicker cross-sections that are more resistant to bending. Unfortunately, moderate distortion may result in expensive and time-consuming inspection and reworking to bring the workpiece within design tolerances. Excessive distortion may lead to scrapping of the workpiece and fabrication of a replacement. Thus, it is also important to know the values of the residual stresses induced by the peening process in order to predict the effect of the residual stresses on the peened workpiece.
Experimental measurement of residual stresses is expensive, time consuming, and can be destructive to the peened workpiece. Non-destructive experimental measurement techniques, such as diffraction methods, can be highly superficial, can limit the size and shape of the measured component due to the inability to capture the diffracted x-rays, can be sensitive to material type (e.g., crystallographic structure), and provide only localized (e.g., point-wise) measurements. Further penetration depth requires at least some localized material removal (e.g., by chemical milling), thus becoming a destructive technique and preventing further in-service use of the measured workpiece.
An example method for predicting residual stresses on a work piece can involve performing an elastic analysis by applying an estimated residual stress field or initial strain field on a model of the workpiece (e.g., typically solved by the finite element method). The predicted residual stress can be obtained by modeling or measurements. Unfortunately, either approach have shown limitations due to difficulty on validating shot peening models and the inability to obtain accurate and representative experimental information at low cost.
Current methods of verifying the accuracy of the estimated residual stress due to shot peening are time-consuming. For example, current methods of verifying residual stress estimates require measuring residual stress in the peened workpiece and comparing the measurements to the predictions from the model solved using the finite element method. The parameters of the shot peening operation are then adjusted or the workpiece may be redesigned in an attempt to adjust the residual stress and distortion in the workpiece. The process is repeated in an iterative manner until the residual stress in the physical workpiece falls within the acceptable limits. Unfortunately, the process of iteratively adjusting the shot peening parameters, fabricating a new workpiece, measuring the residual stress in the new workpiece, and then re-adjusting the shot peening parameters is time consuming and costly.
The peening process (e.g., shot peening or laser peening) relies on test plate coupons (e.g., Almen strips) to calibrate the shot peening intensity. The Almen strip is a small hardened and tempered test strip, which curves on one side when submitted to the intensity of a shot blast stream. Following the peening process, the residual arc height over a fixed length is measured. This measurement defines the peening intensity, generally known as Almen intensity. The peening process is calibrated based on this Almen intensity up to the point that the peening intensity causes saturation on the coupon. Saturation is reached when further peening does not significantly change the arc deformation (e.g., arc height). However, quantifying peening intensity may not be related solely to the residual stress profile.
Thus, quantifying residual stress based on peening intensity measurements may not give accurate information about the residual stress field in the workpiece because different stress profiles may give the same arc height, the height being related to the integral of the residual stress field on the strip thickness. Furthermore, material type is not a characteristic utilized by the Almen strip method. The material used for the coupon is pre-defined (e.g., SAE 1070 spring steel), thus ignoring the actual material of the workpiece (e.g., 6061 aluminum, 7075 aluminum, CRES, Ti64, or any other metal).
Plastic deformation and saturation are directly related to the yield strength of the material of the workpiece. Thus, deformation and saturation for different materials are expected to differ significantly since their elasto-plastic behaviors are different. Therefore, translating peening intensity based on the standard Almen strip method can result in overestimation or underestimation of the residual stresses affecting the peened workpiece. For example, the detrimental effects from exceeding peening intensities (e.g., distortion) can be avoided, while the benefits of higher peening intensities (e.g., higher fatigue life) can be achieved. Due to the continued use of the Almen strip method, it is important to relate the residual stress field induced by shot peening in any mechanical part to Almen intensity.
Given the current state of the art, it is difficult to determine experimentally the complete initial strain field of a shot peened component or part. It is very expensive, time consuming, and error prone to do it for a discrete set of points with the existing experimental methods.
Similarly to the experimental (e.g., measured) results, complete “physics based” mathematical modeling of surface processes (e.g., peening) are extremely complex and difficult to verify and validate. For example, complex models require material parameters that are difficult and costly to obtain. Uncertainty and error propagation for complex physics based models can render the models useless if material data, boundary, and/or initial conditions are not well defined, and numerical verification is not implemented with extreme care (e.g., complex models do not lend themselves to be used by “non-experts”. Moreover, even when using optimal numerical algorithms, these models are so computationally intensive that their use is limited to coupon size problems. Additionally, even if the initial strain field is known in detail, the computation of distortion for large components with high level of detailed information (e.g., spatial variations) may be difficult and computationally expensive.
Accordingly, those skilled in the art continue with research and development efforts in the field of predicting residual stress that occur in a workpiece resulting from a shot peening machine process.